This invention relates generally to printing, and more particularly to a method for halftoning images including multiple colors and multi-level grayscale printing.
Printing with multiple colors or multiple shades of gray uses a halftoning process to convert continuous tone images to a printable format. For instance, 24 bit/pixel continuous tone image data may be converted into 3 or 4 bit/pixel print data. This allows the use of printing technology that imprints ink in fixed quanta (i.e. one or more droplets per pixel may be used.) With only a few actual colors being printable, the perception of a multitude of color tones is created by the combination of adjacent printed pixels.
Halftoning may employ a screen having a matrix of different threshold values. A screen is a data set with each different possible print density value equally represented (or with a controlled unequal distribution for gamma-compensated screens). For monochrome printing, the image data is compared with the screen thresholds at each position, and if the image data exceeds the threshold, a dot is printed; if not, the location remains unprinted. Improved appearance is provided using a pseudo-random stochastic screen having a "blue noise" characteristic. Such screens, as shown in FIG. 1, have the threshold values distributed so that adjacent values tend to be very different, and so that any value or limited range of values will tend to be located at positions that are nicely spaced apart on the matrix, avoid clumping. In this example, a 16-by-16 pixel screen having threshold values in the range of 0-255 is used. This apparently even-but-random spacing is particularly emphasized at the extremely low and high density values in a blue noise screen. For monochrome printing, an example is shown at FIG. 2, in which an even field of 10% cyan is printed, with cyan dots at all locations where the threshold values of the original screen are 25 or less, accounting for 10% of the printable locations.
For printing with multiple colors, halftoning presents a particular challenge. For dot-on-dot printing, in which printed locations are printed with one or more dots, a single halftoning screen is used. For instance, a field of 10% blue would have 10% of locations printed with cyan and magenta ink, while 90% of locations are unprinted. This has the disadvantage of reduced spatial frequency with respect to methods that distribute dots to different locations, and gives the appearance of darker dots, more widely spaced apart, or a "grainy" image. The same applies for clustered dot printing techniques, in which different color dots may be printed adjacent to each other or otherwise clustered to create a multi-dot cluster that reads as an intermediate color. Accordingly, it is preferable to print the individual dots at closely spaced separate (i.e. non-overlapping) locations, relying on the viewer's eye to integrate the different color dots into the intended color.
By using different screens having the threshold values arranged differently, the dots will tend not to align with each other. However, with uncorrelated screens, the printed patterns of different colors will tend to be randomly located with respect to each other, leading to some graininess of the image as some dots happen to clump near others or overlap. With two colors, to reduce this, an "inverted" screen is used for one of the colors, as shown in FIG. 3. An inverted screen has values equal to the maximum screen value, less the screen value at the corresponding location on the other screen. Thus, for the illustrated screen, 10% blue is printed by printing cyan dots at all locations where the threshold values of the original screen are 25 or less, and magenta dots are printed at locations of values of 230 and above on the original screen (25 or less on an inverted screen). Inverted screens are limited in usefulness for several reasons.
First, inverted screens may only be used for two colors, which is inadequate for most multiple color printing systems. Where image quality is not critical in three color (CMY-cyan, magenta, yellow) systems, the darker C and M dots may be printed in this way, while the less visible yellow dots may be distributed otherwise. For four-color systems employing black ink and for multi-level grayscale printing, the inverted screen is inadequate to provide desired image quality. Second, even for two color systems, where (for the illustrated 10% blue example) one color is printed at the lowest value range positions, and another is printed at the highest value range positions, those positions are not relatively well dispersed with respect to each other in a blue noise screen. Although it will not generate overlapping droplets at less than full coverage printing, a high frequency blue noise screen will lead to clumps of adjacent dots, as shown in FIG. 3. Beyond the random effects leading to such clumping, widely different values are more likely to be adjacent to each other.
For three and four color systems, a shifted screen approach has been employed to avoid pure dot-on-dot printing for some colors. This can lead to increased graininess of the image, but more often generates unwanted low frequency artifacts that are visible in the printed image. It is also believed possible that moire patterns may be generated. As illustrated in FIG. 4, the example screen has been used for printing cyan droplets in a 10% blue (10% c, 10% m) pattern. The same screen has been shifted by 8 pixels to the right for locating the magenta droplets (shifted screen not shown), each of which are located 8 pixels to the right of a corresponding cyan droplet. This leads to several visible incidents of droplet overlap, and an unevenly spaced pattern, is causing a grainy appearance.
Accordingly, there is a need for a halftoning process for printing systems employing three or more colors, including multiple shades of gray, that provides an evenly dispersed pattern of individual drops of different colors, and in which the droplets are evenly dispersed relative to all droplets, and not just to those of their own color. In addition, there is a need for a system for evenly distributing color ink droplets when a high density image has greater than full coverage, i.e. when all locations are printed, and at least some are printed by at least two different colors. For such high density printing requirements, the locations with second droplets of each color should be nicely distributed in an even but random appearing manner, without bias toward overlapping a particular color or colors.
The present invention overcomes these disadvantages by providing a method of operating a color printing system that can print any of several different colors, including different shades of gray, on a sheet of printer media on a sheet of printer media.